Fuel pin for a liquid-metal-cooled fast-breeder nuclear reactor

ABSTRACT

An inherently safe fuel pin for a liquid-metal-cooled fastbreeder nuclear reactor is designed so that axial movement of molten fuel is enhanced and return of the molten fuel to the fuel zone is prevented if a sudden reactivity insertion causes melting of the fuel. The fuel pin includes an annular upper blanket separated from a solid fuel zone by a conical nozzle the upper nose of which is welded into a grid plate which extends across the fuel pin and supports the annular upper blanket. Molten fuel is ejected into the central opening of the annular blanket through the conical nozzle which also prevents molten fuel from reentering the fuel zone.

United States Patent [1 1 Neimark June 19, 1973 [54] FUEL PIN FOR ALlQUID-METAL-COOLED 3,619,366 11/1971 Chubb et a1. 176/79 X FASTJ3REEDERNUCLEAR REACTOR 3,145,149 8/1964 Imhoff 176/73 X [75] Inventor: LawrenceA. Neimark, Elmhurst, Ill. Primary Examiner cafl D. Quarforth [73].Assignee: The United States of America as s ista t Examiner-E- E-Lehmann represented by the United States A orney-Roland A- AndersonAtomic Energy Commission, Washington, D.C. 57 ABSTRACT [22] Fil d; J n,28, 1972 An inherently safe fuel pin for a liquid-metal-cooledfast-breeder nuclear reactor is desi ned so that axial [21] Appl' 221745movement of molten fuel is enhance and return of the molten fuel to thefuel zone is prevented if a sudden re- [52] U.S. Cl. 176/68, 176/17,176/74, activity insertion causes melting of the fuel. The fuel 176/77,176/40 pin includes an annular upper blanket separated from [51] Int.Cl. G2lc 3/18 a li f el zo y a nical nozzle the upper nose of [58] Fieldof Search 176/40, 64, 73, 74, whi h i welded into a grid plate whichextends across v 176/68, 77, 83, 17, 18 the fuel pin and supports theannular upper blanket Molten fuel is ejected into the central opening ofthe [56] Ref ren Cit d annular blanket through the conical nozzle whichalso UNITED STATES PATENTS prevents molten fuel from reentering the fuelzone.

3,362,882 1/1968 Sofer et al.- 176/18 4 Claims, 4 Drawing Figures FUELPIN FOR A LIQUID-METAL-COOLEI) FAST-BREEDER NUCLEAR REACTOR CONTRACTUALORIGIN OF THE INVENTION The invention described herein was made in thecourse of, or under, a contract with the UNITED STATES ATOMIC ENERGYCOMMISSION.

BACKGROUND OF THE INVENTION This invention relates to an inherently safefuel pin for a liquid-metal-cooled fast-breeder reactor. In more detailthe invention relates to a fuel'pin in which the consequences ofoverpower transients or fuel pin failures are precluded or mitigated.The invention also relates to a fuel pin in which irreversible movementof fuel results from fuel melting. In still more detail the inventionrelates to a fuel pin incorporating means for enchancing axial movementof molten fuel and for preventing molten fuel from reentering the coreregion if a reactivity addition or increase in power should cause fuelmelting.

Before liquid-metal-cooled fast-breeder reactors can be commerciallypracticable, it must not only be certain that no conceivable accidentcould cause a release of radioactivity to the environment but also thatrelatively minor reactivity additions or increases in power will notcause the reactor to be out of service for an extended period of time.Thus the purpose of an inherently safe fuel element design for afast-breeder reactor is to preclude or mitigate the consequences ofoverpower transients or fuel element failures. Serious accidents due toincreased reactivity can be mitigated by moving fuel axially away fromthe core midplane. It has been shown that the energy transient requiredto cause failure of an irradiated oxide fuel pin can be significantlyincreased if the fuel pin will accommodate a volume increase of moltenfuel and will provide internal pressure relief. This can be accomplishedby providing an annular blanket above the fuel. See Report No.GEAP-l3543 by T. Hikido and .I. H. Field. This report is available fromthe Clearing House for Federal Scientific and Technical Information,National Bureau of Standards, U. S. Department of Commerce, Springfield,Vir.

Movement of fuel axially within the fuel pin not only relieves thepressures which might cause the fuel pin to fail due to radial expansionof fuel but also reduces reactivity since fuel moves from a high worthposition in the reactor core to a position of lower worth. Melting ofthe fuel followed by axial movement thus inherently causes a reductionin reactivity. However, molten fuel after having moved axially up intothe upper blanket could then drop by gravity back into a high worthposition in the reactor, causing an increase in reactivity. Such anincrease, of course, could not be. tolerated.

SUMMARY OF THE INVENTION Accordingly, means for enhancing axial movementof molten fuel and for preventing the molten fuel from reentering thefuel zone of the fuel pin is provided in a fuel pin containing anannular upper blanket. This consists of a conical nozzle having its noseup, the nozzle causing the molten fuel to be ejected up into the blanketif an accident occurs and catching the molten fuel should it fall backin the fuel pin. A support grid for the annular blanket into which thenose of the conical nozzle is welded is provided at the top of theconical nozzle.

SPECIFIC EMBODIMENT OF THE INVENTION The invention will next bedescribed with reference to the accompanying drawing wherein:

FIG. 1 is a side elevational view of a fuel pin for a fast-breedernuclear reactor,

FIG. 2 is a vertical cross section of a portion thereof wherein islocated a device for enhancing axial movement of molten fuel and forensuring that the movement is irreversible,

FIG. 3 is a horizontal cross section taken on the line 3-3 of FIG. 2,and

FIG. 4 is a partial cross section taken on the line 4-4 of FIG. 3.

As shown in FIG. 1, a fuel pin for a fast-breeder reactor includes afuel zone 10, a lower blanket 11, an upper blanket 12, a plenum 13, andan insulator zone 14, in addition to upper end cap 15 and lower end plug16. In accordance with the present invention, a concical nozzle 17 isincluded between the upper blanket l2 and the fuel zone 10 and thisfeature will next be described in connection with FIGS. 2, 3 and 4'ofthe drawing.

In more detail, the fuel comprises a cladding tube 18 enclosing aplurality of solid cermaic fuel pellets l9 constituting fuel zone 10 anda plurality of annular ceramic blanket pellets 20 constituting upperblanket 12 in addition to lower blanket 11. Conical nozzle 17 is locatedbetween fuel zone 10 and upper blanket 12 and includes a nose 21 whichis welded into the central opening of a grid plate 22. Grid plate 22extends completely across cladding tube 18 and serves to support upperblanket 12. It will be observed that the grid plate 22 is slightlydeformed to accommodate the conical nozzle. The lower, larger end ofconical nozzle 17 abuts the cladding tube 18 to form a pocket 23 betweenthe conical nozzle and the cladding tube. Chamber 24 is provided toenlarge axial opening 25 in the lowermost, upper blanket pellet 20 toopen up pocket 23 to the axial opening in the upper blanket pellets.

According to one specific embodiment of the invention, the cladding tubeis formed of stainless steel 0.25 inch in diameter, fuel zone 10 is 36inches long, lower blanket 11 is 12 inches long, upper blanket 12 is 12inches long and the axial opening 25 therein is 0.070 inch in diameter,plenum I3 is 3 feet long, insulator zone 14 is 0.5 inch long, andconical nozzle 17 is formed of stainless steel and is 3 inches long. Thelower end of nozzle 17 fits snugly within cladding tube 18 and thediameter of the nose is 0.070 inch, the aperture therein being 0.040inch in diameter. In addition, grid plate 22 is formed of two parallelstainless steel plates disposed at right angles to two other parallelstainless steel plates. The plates are 0.015 inch in thickness, have aheight of 0.062 inch and the distance between plates is 0.062 inch.Finally, the fuel employed is mixed UO -PuO and the blanket material isdepleted U0 Fuel pins of the type described above may be located atcertain selected locations within a fast reactor core or all fuel pinsmay be constructed in this way. Detailed analysis for each reactor willbe necessary to determine the optimum arrangement of these fuel pins.The fuel pin described herein has been designed specifically for aliquid-metal-cooled reactor; however, the invention described herein canbe equally well applied to fuel pins for gas-cooled reactors.

As pointed out in the above-cited report, the sudden release of fissiongases during a transient which results in fuel melting will give rise tosubstantial internal pressures and will provide a substantial drivingforce for fuel relocation and dispersion. Thus, should the fuel in afuel pin constructed in accordance with the present invention melt, thefuel will be ejected upwardly through conical nozzle 17 into the centralhole in annular blanket l2 and the distance travelled by the molten fuelis greater than it would be in the absence of the nozzle. ln addition,the open structure of grid 18 and the small aperture in the nose ofconical nozzle 17 result in the trapping in pocket 23 formed between theconical nozzle 17 and the cladding tube 18 of a substantial proportionof any molten fuel that subsequently flows downwardly out of the annularblanket. Trapping the fuel thus prevents the fuel from reentering thefuel zone and causing a reactivity increase. Thus provision of a conicalnozzle in accordance with this invention ensures that fuel movement isfast, the fuel is moved a substantial distance and the movement isirreversible.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a fuel pin for a fast-breeder reactor incorporating cermaic fueland an upper annular ceramic blanket supported by a grid plate, theimprovement comprising a conical nozzle having its nose welded into saidgrid plate disposed between the ceramic fuel and the annular blanket forenhancing axial movement of molten fuel and for ensuring that this axialmovement is irreversible if a re-activity insertion or increase in powercauses fuel melting.

2. A fuel pin for a liquid-metal-cooled, fast-breeder reactor comprisinga cladding tube, a plurality of ceramic fuel pellets constituting a fuelzone disposed in said cladding tube, a plurality of annular ceramicblanket pellets supported by a grid plate constituting an upper blanketdisposed in said cladding tube above said fuel zone, and a conicalnozzle having its nose welded into said grid plate located in thecladding tube between the upper blanket and the fuel zone for enhancingaxial movement of molten fuel and for ensuring that this axial movementis irreversible if a reactivity insertion or increase in power shouldcause fuel melting.

3. A fuel pin according to claim 3 wherein the ceramic fuel is UO PuOand the blanket pellets are depleted U0 4. A fuel pin according to claim3 wherein the cladding tube is formed of stainless steel 0.25 inch indiameter, the conical nozzle is formed of stainless steel, the conicalnozzle is 3 inches long, the outer diameter of the noze thereof is 0.070inch and the aperture therein is 0.040 inch in diameter, the centralopening in the annular blanket is 0.070 inch in diameter, and the inneredge of the lowermost annular blanket pellet is cham-

2. A fuel pin for a liquid-metal-cooled, fast-breeder reactor comprisinga cladding tube, a plurality of ceramic fuel pellets constituting a fuelzone disposed in said cladding tube, a plurality of annular ceramicblanket pellets supported by a grid plate constituting an upper blanketdisposed in said cladding tube above said fuel zone, and a conicalnozzle having its nose welded into said grid plate located in thecladding tube between the upper blanket and the fuel zone for enhancingaxial movement of molten fuel and for ensuring that this axial movementis irreversible if a reactivity insertion or increase in power shouldcause fuel melting.
 3. A fuel pin according to claim 3 wherein theceramic fuel is UO2-PuO2 and the blanket pellets are depleted UO2.
 4. Afuel pin according to claim 3 wherein the cladding tube is formed ofstainless steel 0.25 inch in diameter, the conical nozzle is formed ofstainless steel, the conical nozzle is 3 inches long, the outer diameterof the noze thereof is 0.070 inch and the aperture therein is 0.040 inchin diameter, the central opening in the annular blanket is 0.070 inch indiameter, and the inner edge of the lowermost annular blanket pellet ischamfered to enlarge the opening.